Radiation imaging systems are widely used for medical and industrial purposes, such as for computer tomography (CT) and luggage scanning. Imaging systems have been developed that use detected radiation to produce a signal which can be used to operate a visual display device or which can be used for other analyses of the pattern of detected radiation such as x-ray or gamma radiation. In such systems the radiation is typically absorbed in a scintillator material, resulting in generation of photons of light. Light photons emanating from the scintillator are detected by photodetectors to generate an electrical output signal that can be processed to drive the display or analysis system.
Scintillator arrays or blocks heretofore have been composed of several individual crystal elements assembled with reflectors between the crystal elements. The crystal elements were optically coupled to PIN diode arrays to form individual detector arrays composed of multiple channels (a crystal element and photodiode together form a "channel" that produces an output for processing along with the outputs of the other "channels"). These detector arrays were then placed in groups of several arrays such that the resulting matrix acts as a single array.
One known method of forming a scintillator array involved attaching a reflector to each internal surface of the crystal elements and then bonding them together. The outside surfaces of the thus bonded assembly were then covered with reflector material to finish the array.
Another method involved the use of the bonding material as the reflector between the individual crystal elements. One such bonding material was Eccobond 45 clear epoxy adhesive available from Emerson & Cuming, Inc. of Woburn, Mass., USA. The Eccobond 45 mixed with a catalyst 15 clear curing agent or hardener had an amber color. To improve the reflective properties of the epoxy adhesive, the Eccobond 45 was doped with titanium oxide with this yielding an off-white color interface material that was suitable for use in forming the crystal array. The outside surfaces of the bonded assembly were then covered with a reflective paint to finish the crystal array. In the resulting detector array (the crystal array with the photodiodes assembled thereto), the variation in outputs of each channel was good, i.e., within .+-.5%.
The detector arrays formed from crystal blocks using the Eccobond 45 were found to be unacceptable because of high expansion rates in applications where the detector arrays encountered large temperature excursions including those that crossed the glass transition temperature of the epoxy adhesive. The problem arose from the relatively low thermal expansion of the crystal element composed, for example, of CdWO.sub.4 compared to the epoxy adhesive, and the fact that the bond between the epoxy and adjacent surfaces of the crystal element restrict the volumetric expansion of the epoxy interface layer in two geometric dimensions such that most expansion occurs in the third dimension. The result is possible damage to the detector arrays and unacceptable stress on the optical coupling between the crystal elements and photodiodes.
A discovery was made of another epoxy that had an expansion rate closer to that of the scintillator crystal. This epoxy was Eccobond 24 water clear epoxy adhesive available from Emerson & Cuming, Inc. when mixed with Catalyst 9 instead of the intended Eccobond 24 Part B catalyst. The Eccobond 24 was doped with titanium oxide to produce a white material having a lower expansion rate, higher glass transition temperature and higher reflectivity than the above mentioned doped Eccobond 45 epoxy adhesive. Those skilled in the art would view the higher reflectivity as being desirable in that it increases the number of photons that are detected by the photodiode. However, it was found that the end channels of the detector arrays generally had a substantially lower output than the inner channels. This phenomena also has been found to occur in other detector arrays using different reflectors than the aforesaid low expansion rate epoxy adhesive. Typical variations in output of .+-.15% are not uncommon in the industry.
The present invention provides a solution to the problem of low end channel output in detector arrays that otherwise would exhibit such phenomena. According to the invention, the output variations of individual channels and particularly the outer channels of a scintillator crystal or detector array are reduced by modifying the reflectivity of the surfaces of the individual crystal elements according to the output. This is accomplished by creating a specific difference in the reflectivity of the internal channel reflectors compared to the external reflector material to enhance or reduce the output of specific channels to achieve a balance in the outputs. The specific difference in reflectivity may be obtained by using different color pigments, different color cements and various colored foils, for example.
According to one aspect of the invention, there is provided a scintillator array comprising plural scintillator elements assembled side-by-side with inner reflectors interposed between the sides of relatively adjacent scintillator elements, and outer reflectors at the outermost sides of the outer scintillator elements between which one or more inner scintillator elements are sandwiched. The inner reflectors include an epoxy adhesive adhered to the sides of the relatively adjacent scintillator elements, the epoxy adhesive has a glass transition temperature greater than 80.degree. C, and the inner reflectors and outer reflectors have respective reflectivities providing an output variation between the outer scintillator elements and the inner scintillator elements of no greater than about .+-.10%, and more preferably no greater than about 5%.
In a preferred embodiment, the outer reflectors are formed by a reflective coating, for example a paint, on the outermost sides of the outer scintillator elements which are scintillation crystals. The inner reflectors further include a pigment in the epoxy adhesive to increase the reflectivity of the epoxy adhesive, and the epoxy adhesive is formed from an epoxy resin and two catalysts, one catalyst being clear and composed at least in part of tetraethylene pentamine, triethylene tetramine and pentaethylene hexamine, and the other catalyst being colored and composed of m-phenylene diamine and n-methyl pyrrolidone. Plural photodiodes may be optically coupled to the scintillator elements to form a detector array.
According to another aspect of the invention, there is provided a method for compensating between variations in outputs of individual scintillator elements of a scintillator array wherein the scintillator elements are assembled together side-by-side with inner reflectors interposed between the sides of relatively adjacent scintillator elements and outer reflectors at the outermost sides of the outer scintillator elements between which one or more inner scintillator elements are sandwiched. The method is characterized by the step of modifying the reflectivity of the reflectors to reduce any output variation between the scintillator elements to no greater than about .+-.10%.
According to a preferred method, the modifying step includes using different color reflective materials for the inner and outer reflectors. The preferred method further includes the step of using an epoxy adhesive to form the inner reflectors and further to bond together the relatively adjacent scintillator elements. The epoxy adhesive preferably has a glass transition temperature greater than 80.degree. C., and the modifying step may include selecting the inner and outer reflectors such that they have respective reflectivities providing an output variation between the outer scintillator elements and the inner scintillator elements of no greater than about .+-.5%. The method also may include the step of applying a reflective coating to the outermost sides of the outer scintillator elements to form the outer reflectors, and the epoxy adhesive may be formed from an epoxy resin and a first catalyst composed at least in part of tetraethylene pentamine, triethylene tetramine and pentaethylene hexamine and a second catalyst composed of m-phenylene diamine and n-methyl pyrrolidone.
The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail a certain illustrative and preferred embodiment of the invention, this being indicative, however, of but one of the various ways in which the principles of the invention may be employed.